1. Field of the Invention
The present invention relates to a closed battery capable of rupturing at a fixed pressure, especially at a low pressure, to readily release the internal pressure and at the same time to cut off the current, when the temperature or the internal pressure of the battery is elevated.
2. Description of Related Art
Recently, with increasing demand for electronic devices that are made much smaller in size and have enhanced performance, non-aqueous electrolyte batteries with light weight and high energy density such as lithium ion secondary battery have been popularly used. Since the non-aqueous electrolyte battery uses an alkali metal such as lithium, sodium, or potassium as an active material of negative electrode thereof and such alkali metal easily reacts with water to generate gases, it employs a container of completely sealed structure. Such a completely closed battery is excellent in storability, but reversely, there arises a problem due to the very high sealability. Namely, if the battery is exposed to high temperature or is short-circuited inside or outside thereof and a large current flows, or if hydrogen gas is generated in the battery due to the moisture included in the manufacturing process, the internal pressure of the battery is sometimes abnormally elevated and the battery bursts, which may damage the devices. There may be also a danger of injuring the user of the device. Accordingly, it is necessary to provide some mechanism for releasing internal pressure of the battery before it is increased to a high extent.
So far, several methods for releasing the internal pressure of a battery have been proposed and some of them have been used in practice. For instance, Japanese Patent Laid-open Publication No. HEI-2-304861 discloses a mechanism of a safety valve for a battery. This safety valve is constructed such that a battery container is provided on a portion thereof with a valve chamber having a valve opening leading to the inside of the battery and an exhaust hole leading to the outside. The valve chamber is provided with a valve member, in which at least the surface facing the valve opening is made of rubber, and an elastic member to press rubber-made surface of said valve member against the valve opening. Thus, the rubber of the valve member closes the valve opening so that the sealability of the battery can be ensured, while the valve opening is unsealed when the internal pressure reaches a fixed pressure for the elastic member to permit pressure releasing so that excessive elevation of the internal pressure can be prevented.
Such a conventional safety valve has been widely used in Nixe2x80x94Cd type batteries. However, it has not yet been used in the non-aqueous batteries which employ alkali metals as the active material of the negative electrode, since even if the elastic member presses the valve member against the valve opening to thereby close the battery, it is impossible to obtain such a high sealability as required for the non-aqueous batteries.
One of the methods for releasing the internal pressure of a battery which requires high sealability is disclosed in Japanese Patent Laid-open Publication No. SHO-63-285859. In this method, a container of the battery is provided at a portion of its wall with a thinned portion. This thinned portion is formed by cold compressing the can wall sheet using a pressing machine until the thickness of the compressed portion of the sheet becomes half of the original thickness thereof. Thus, this thinned portion can provide the battery with good sealability since a portion of a can body having no opening on it is only transformed.
Another pressure releasing mechanism of this type is disclosed in Japanese Patent Laid-open Publication No. HEI 6-215760. This releasing mechanism is provided with a valve diaphragm fitted in an open end portion of at the upper portion of an electrode element in a cylindrical outer container having a bottom and a lead for cutting-off the current disposed above the valve diaphragm. In this structure, when the internal pressure of the battery is elevated, the valve diaphragm is expanded to rupture the lead for cutting-off the current to break off the current.
According to the method as disclosed in Japanese Patent Laid-open Publication No. SHO-63-285859, it is necessary to thin off a thinned wall portion of the container so that the thinned portion can be ruptured at a relatively low pressure. However, when the wall portion is too thinned off, there may be formed fine or minute cracks during press-forming, which may impair the sealability of the battery container. Further, when metals are processed by cold working, they are inevitably hardened by working. However, the working does not always cause uniform hardening, and therefore, there arises a problem that the operating pressure for such pressure releasing mechanism may vary. There is a half-etching method for making the thinner portion of the wall of a battery container. However, it is extremely difficult to control the remaining thickness of the thinned portion remained after etching, and it is also difficult to obtain a good yield. Another problem is that pinholes are generated in the half-etched portion, and therefore, all the products need to be inspected.
According to the method as disclosed in Japanese Patent Laid-open Publication No. HEI-6-215760, it is necessary to accurately control the depth of a cut off portion formed in the lead. This method is difficult in view of its forming, which cause a problem that the current is not reliably cut off at a fixed pressure.
With a view to solving drawbacks of the conventional methods as mentioned above, it is an object of the present invention to provide a closed battery incorporating a valve chip which can be stably and accurately ruptured at a fixed pressure, especially at a low pressure, so as to release the internal pressure thereof and which can be manufactured easily, and a closing member for use in the above mentioned closed battery. It is also the object of the present invention to provide a closing member having a configuration capable of being easily handled and almost free from causing defective products in the manufacturing process, and a closed battery using the closing member.
The above mentioned problem can be solved according to the present invention by a closed battery comprising an outer container, an electrode element consisting of a positive electrode, a negative electrode, and a separator, accommodated in said outer container together with an electrolyte, and a closing member secured to the inner periphery of an open end portion of the outer container so as to close the open end portion. The closing member consists of a valve element and a metal foil, and the valve element is formed of a metal substrate, on which a plurality of perforated openings are disposed along an endless circumference with small interval between each opening. The metal foil is laminated to the inner surface of the metal substrate so as to close the perforated openings. When the internal pressure of the battery is elevated, portions of the small interval between the openings formed on the metal substrate, which are transformable by a small force, are elongated so as to cause a portion of the metal substrate encircled by the plurality of perforated openings disposed on the endless circumference to be projected.
According to the configuration of the present invention, a valve chip, which is composed of the portion of the metal substrate encircled by the plurality of perforated openings and the metal foil, is connected with a lead cover for conducting a current from the electrode element. When the internal pressure of a battery is elevated, the portion encircled by the plurality of perforated openings is projected, and the lead cover is electrically disconnected from the metal foil to cut off the current. When the internal pressure is further elevated, the metal foil, which is laminated to close the openings of the valve element formed of the metal substrate having the perforated openings thereon, ruptures to release the internal gas of the battery so as to prevent burst of the battery container.
The above mentioned plurality of perforated openings are circular openings having the same diameter. They are preferably be disposed with equal intervals respectively along a circumference of a fixed radius around the center of the valve element, which is an example for the endless circumference. Further, the sum total length of the openings formed as the plurality of perforated openings on the endless circumference is preferably 30 to 85% of the whole length of the endless circumference.
Furthermore, the metal substrate is preferably a steel sheet, a stainless steel sheet, a copper sheet, or an aluminum sheet. The metal foil is preferably a steel foil, a stainless steel foil, a copper foil, an aluminum foil, a nickel foil, or a nickel-iron alloy foil.
Thus, the closing member of the present invention is constructed such that a portion thereof is transformed at a fixed pressure to rapidly cut off the current of the battery when the internal pressure is elevated. The closing member is further provided with a mechanism for ruputuring itself at a fixed pressure to release the internal pressure. The above mentioned fixed pressure for cutting off the current can be set arbitrarily according to the shape and disposition of the openings, and the pressure value at the internal pressure released can be set arbitrarily according to the material and thickness of the metal foil.